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(SAR) The Specific Absorption Rate is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The SAR describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal. Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the concept of a local SAR. Hot spots may occur in the exposed tissue, to avoid or at least minimize effects of such theoretical complications, the frequency and the power of the radio frequency irradiation should be kept at the lowest possible level. Averaging over the whole body leads to the global SAR.
It increases with field strength, radio frequency power and duty cycle, transmitter-coil type and body size. The doubling of the field strength from 1.5 Tesla (1.5T) to 3 Tesla ( 3T) leads to a quadrupling of SAR. In high and ultrahigh fields, some of the multiple echo, multiple-slice pulse sequences may create a higher SAR than recommended by the agencies. SAR can be reduced by lower flip angle and longer repetition times, which could potentially affect image contrast.
Normally no threatening increase in temperature could be shown. Even in high magnetic fields, the local temperature increases not more than 1°C. 2.1°C is the highest measured increase in skin temperature. Eddy currents may heat up implants and thus may cause local heating.
FDA SAR limits:
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Whole body: 4W/kg/15-minute exposure averaged;
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Head: 3W/kg/10-minute exposure averaged;
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Head or torso: 8W/kg/5 minute exposure per gram of tissue;
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Extremities: 12W/kg/5 minute exposure per gram of tissue.
IEC (International Electrotechnical Commission) SAR limits of some European countries:
All limits are averaged over 6 minutes.
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Level 0 (normal operating mode): Whole body 2W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg;
Extremities (local) 20W/kg;
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Level I (first level controlled operating mode): Whole body 4W/kg; Head 3.2W/kg; Head or Torso (local) 10W/kg; Extremities (local) 20W/kg;
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Level II (second level controlled operating mode): All values are over Level I values.
(For more details: IEC 60601-2-33 (2002))
In most countries standard MRI systems are limited to a maximum SAR of 4 W/kg, so most scanning in level II is impossible.
For Level I, in addition to routine monitoring, particular caution must be exercised for patients who are sensitive to temperature increases or to RF energy.
For Japan different SAR limits are valid. | | | | | | | | | Further Reading: | | Basics:
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Developed by GE Lunar; the ARTOSCAN™-M is designed specifically for in-office musculoskeletal imaging. ARTOSCAN-M's compact, modular design allows placing within a clinical environment, bringing MRI to the patient. Patients remain outside the magnet at all times during the examinations, enabling constant patient-technologist contact. ARTOSCAN-M requires no special RF room, magnetic shielding, special power supply or air conditioning.
The C-SCAN™ (also known as Artoscan C) is developed from the ARTOSCAN™ - M, with a new computer platform.
Device Information and Specification
CLINICAL APPLICATION
Dedicated extremity
SE, GE, IR, STIR, FSE, 3D CE, GE-STIR, 3D GE, ME, TME, HSE
SLICE THICKNESS
2D: 2 mm - 10 mm;
3D: 0.6 mm - 10 mm
4,096 gray lvls, 256 lvls in 3D
POWER REQUIREMENTS
100/110/200/220/230/240V
| | | | • View the DATABASE results for 'ARTOSCAN™ - M' (3).
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| | | | • View the DATABASE results for 'Absorbed Dose' (2).
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The term 'active' refers to any medical device that can only serve its intended use with the supply of power by any means including, but not limited to line, battery, or gas power. Examples of active devices include ventilators, pacemakers, and patient monitoring devices. | | | | • View the DATABASE results for 'Active Device' (4).
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| | | | • View the DATABASE results for 'Amplifier' (15).
| | | • View the NEWS results for 'Amplifier' (2).
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